When to omit だ when answering questions?

Question

Maybe this is a stupid question, but in Minna no Nihongo it says to omit だ when giving a positive answer to a question.

Question: こんばんひま？
Answer: うん、ひま。

But it is not explained if one can omit だ when answering this way:

Example 1

Question: あなたはにほんじん？
Answer: ううん、ちゅうごくじん (だ?)。

Example 2

Question: あなたのしゅみはなに？
Answer: にほんご (だ?)。

Do I have to put だ at the end of these two answers or should I omit it?

Answer 1

Grammatically in your cases using or omitting だ does not make any difference.

But Japanese speakers use specific forms in specific situations, and it changes when the relationship between speakers changes.

In both example, the question sentence and ううん is quite casual, and sounds like the speakers are female. Using a single だ for the end of a sentence is strong and mostly used by male speakers, so I will omit it here. If you want to use だ, adding another sentence ending particle after it will be more natural. Ex. にほんごだよ

Answer 2

Most of the time "だ" can be either included or omitted when you are talking about yourself.

• うん、ひまだ / うん、ひま
• ちゅうごくじんだ / ちゅうごくじん
• にほんごだ / にほんご

are all fine.

When you say にほんごだ, it emphasises the fact that you speak Japanese. Though it sounds a little bit childish and it is not appropriate to older people. Putting "です" is always a good idea since it sounds polite.